Assuming SE is the same as its big brother.
Just set the main particle number to zero where you want it to end.
Conversely drop the opacity to zero as well, to completely shut it of visually....or both.

Select the "active" graph in the hierarchy window. In the graph window, click on the graph at frame 70. You should now see the graph "end" at frame 70. The emitter and all of its particles will disappear at frame 70.

If you don't want all of the particles to disappear, but instead just die out normally, use the "number" property instead. Select it in the hierarchy, then in the graph click to add a data point at around frame 70. You can drag it down to the value you want, which is "0". Then click to add another data point to the left of that point, and slide it up so that the "number" graph is flat, but then steps down to 0.